Techniques and protocols for assays involving immunoreaction of antigen/immunoglobulin moieties or other specifically binding moieties have been known for almost thirty years. One of the earliest disclosures of such protocols was the original radioimmunoassay procedure of Yalow. There are hundreds of specific assays and dozens of general protocols involved in such assays, whose unifying feature is that their specificity depends on the ability of the analyte in question, but not the contaminants accompanying it in the sample, to bind to the reagent.
Most frequently, the specific binding is an antigen/antibody immunoreaction and its variants, including the use of derivatives of immunoglobulins to bind to an antigen. The antigen, itself, may also of course, be an immunoglobulin. Other examples of specific binding, while perhaps less common, are nevertheless important, including the highly specific reaction between avidin and biotin, the attraction of particular lectins for certain proteins or glycoproteins, and so forth. The specificity need not be exquisite, as long as there are no interfering substances present in the sample which also bind to the reagent.
In broadest concept, the specific binding assays to which the designed reagents of the invention are applicable are conducted either in a direct or competitive manner. In a direct assay, the specific reagent is either itself labeled or provided a mechanism to acquire a label and used to bind to the analyte in the sample, thus removing it from the physical environment of the contaminants, or in some way changing its environment so that only its presence, among the associated materials in the mixture, is detectable. The amount of label associated with the analyte is then a direct measure of the quantity of analyte in the sample. Often a labeled antibody is employed for direct assays, in which case the assay is generally termed "immunometric".
Conversely, in the competitive approach, the analyte is caused to compete with itself in labeled form for the same specific reagent. The higher the concentration of competing analyte in the solution, the less label will be bound to the specific reagent. Thus, the amount of label associated with the specific reagent/analyte complex is in inverse proportion to the amount of analyte in solution.
The foregoing methods can be conducted as solid-phase assays, including sandwich assays, and may involve more than one specifically interacting substance in forming the final labeled conjugate.
Over the more than twenty years that this assay approach has been used, a number of labeling systems have come into common use, depending on the nature of the analyte, and the sensitivity required. The most common labels are radioisotopes, fluorescent materials, or enzymes capable of catalyzing detectable reactions. Radioimmunoassay (RIA), which employs radioisotopes as labels, generally is quite sensitive but, of course, is cumbersome to conduct due to the dangers associated with handling radioisotopes and the equipment involved in quantifying the radiation. Fluorescent labeling has been moderately less sensitive, a problem which might be overcome by increasing the number of dye molecules used as label, were it not for crowding effects resulting in the quenching of fluorescence due to the proximities of the fluorophores. Enzyme-mediated immunoassay techniques are also limited in sensitivity by the number of labeling enzyme molecules which can be crowded onto the specific reagent. The problem of fluorescence quenching with multiple labels is serious enough to diminish the sensitivity of assays using this approach. Concentration quenching with multiple labels is discussed in, for example, Smith, D. S., et al, Ann Clin Biochem (1981) 18:253. Thus, it would be desirable to utilize a method of labeling which permits enhancement of sensitivity by permitting multiple labels, which still retain their effectiveness, to be attached to the specific reagent. The present invention, by designing a method and final product to achieve efficient multiple labeling, provides the opportunity to maximize the practicality of labeled specific analytical binding reactions.